1. Field of the Invention
The invention in general relates to underwater target identification systems, and more particularly to a system for detecting and classifying a resonant target.
2. Description of the Prior Art
Very often when searching for underwater targets such as mines or the like, use is made of a relatively high frequency side-looking sonar apparatus. Basically, the side-looking sonar apparatus travels over a target area under surveillance, typically the sea bed, and in the course of travel periodically projects acoustic pulses toward the target area and receives returns from extremely narrow strips on the target area. After a plurality of transmissions and receptions, a map of the target area, and targets on it, may be built up and with almost photographic qualities so as to permit identification of various targets.
The problem sometimes arises, however, in that the target may not lie on the sea bed. For example, over the course of time a target may become buried, in which case, the high-frequency signal utilized in the side-looking sonar arrangement may not permit detection and classification of the buried target. Although a lower frequency acoustic signal may be utilized for penetration of the sea bed, any target therein would just be detected and not necessarily classified. For example, an acoustic return from a rock may be indistinguishable from that of a mine.